1. Field of the Invention
The present invention relates to a radio receiving apparatus with an adaptive receive filter.
2. Description of Related Art
A receiving apparatus in wireless communication, may receive signals on which signals from channels other than the communicating channel are superimposed, which is referred to as “interference.” When interference is caused, unless interference waves, which are an unnecessary signal with respect to a desired signal to be demodulated essentially, are canceled, demodulation characteristics deteriorate substantially. Therefore, it is necessary to cancel interference waves using a filter of a relatively narrow band and transfer a signal having high signal quality (referred to as “SNR”) to later processing on the assumption that a band necessary for a signal of each communication standard and channel separation are wide. To perform the above-described processing, a receiving apparatus may include a receive filter. As this receive filter, a receive filter having a single frequency characteristic can be used as long as desired reception performance is satisfied. However, depending on communication standards, it may be difficult to cancel interference wave and secure necessary band with a single receive filter. That is, when a single narrow-band receive filter is employed to cancel interference waves, a frequency band of the received signal is cut too much, and, as a result, reception performance for the case where there is no interference wave may not be satisfied. Alternatively, when a receive filter of a relatively wide band is employed to satisfy reception performance for the case where there is no interference wave, interference waves cannot be canceled sufficiently, and, as a result, reception performance for the case where there are interference waves may not be satisfied. In this case, there is a measure of changing the types of the receive filter depending on whether or not there are interference waves, for example, making coefficients variable, or adopting a receive filter configuration providing a plurality of filters such as filters for interference cancellation and filters used when there is no interference wave, and selectively adopting a filter. As a receive filter which selectively adopts a plurality of filters, there is, for example, a reception filtering method disclosed in Patent Document 1 (Japanese Patent Application Laid-Open No. 2004-336536).
FIG. 1 is a block diagram showing the configuration of the radio receiving apparatus with the adaptive receive filter disclosed in Patent Document 1.
In FIG. 1, radio receiving apparatus 10 is configured with antenna 11, PGA (Programmable Gain Amplifier) 12, A/D converter 13, adaptive receive filter section 20, filter selecting section 21 and modulation scheme determining section 22.
The reception filtering method of radio receiving apparatus 10 determines a modulation scheme from known information, which is included in the received signal per predetermined processing unit called “time slot.”
However, whether or not there are interference waves does not depend on the modulation scheme, and so this conventional reception filtering method cannot decide correctly whether or not there are interference waves, and reception performance may deteriorate as a result of cutting the signal band more than necessary, or reception performance may deteriorate as a result of insufficient interference wave cancellation. That is, the point in the configuration of the filter-selective receive filter is that whether or not interference waves can be detected at high accuracy.
Further, the intersymbol interference canceling method disclosed in Patent Document 1 determines the modulation scheme of a received signal and determines whether or not to adopt an intersymbol interference canceling filter based on that information. However, the intersymbol interference canceling filter generally has a characteristic of lifting the signal level toward a high frequency band. Therefore, when the intersymbol interference canceling filter is simply adopted, the out-of-band noise level increases, which leads to signal quality (SNR) deterioration, and, as a consequence, reception performance deteriorates. Therefore, when an intersymbol interference canceling filter is adopted, reception performance cannot be improved without judging the out-of-band noise level quantitatively and deciding whether or not to adopt the intersymbol interference canceling filter based on that result.
In this way, with a radio receiving apparatus having an adaptive receive filter, when channel separation is narrower than the signal bandwidth, it is difficult to cancel adjacent interference and secure signal band with one receive filter. Further, there are requirements for the noise level when an intersymbol interference canceling filter is adopted, and it is difficult to decide the noise level correctly from the received signal.